Additives such as pigments, dyes, stabilizers, catalysts, flame retardants and antimicrobial agents are often mixed with molten thermoplastic polymers prior to extruding the polymer into a shaped article. These additives may be in the form of concentrated dispersions of additive in a carrier polymer, the carrier polymer being compatible with the thermoplastic polymer to be extruded. There are several known methods of introducing additive concentrates to a thermoplastic polymer.
One method is to meter a pelletized additive concentrate into the throat of an extruder using either gravimetric or volumetric feeders. This method is costly, difficult to control and results in a large amount of waste polymer when transitioning between additive concentrates.
Another method of introducing additive concentrates into a thermoplastic polymer is to first melt the concentrate and then meter the liquid concentrate into the molten thermoplastic polymer. This method is easy to control and can result in less waste polymer when transitioning between additive concentrates. However, the cost of melt addition equipment is high.
Another method of introducing additives into a thermoplastic polymer is to meter the additives in the form of concentrate strands to a cutting means which forms additive pellets which are then gravity fed into an extruder. Such processes are described in U.S. Pat. Nos. 3,530,754; 4,848,915; 4,919,872 and German Patent No. DE 3523661. These processes are most versatile when more than one additive concentrate strand, containing different additives, are simultaneously metered to a cutter to form pellets which are fed to an extruder. However, it is difficult to accurately control the simultaneous feeding of several strands at different rates to the cutter in these processes. It is especially difficult when the feed rate of one strand is much slower than the feed rate of the other strand(s). The slower moving strand is cut into light weight "fines" rather than pellets. These fines are not easy to gravity feed into the extruder and the additive level in the extruded product may be unacceptably non-uniform with time.
Accordingly, it is an object of this invention to accurately control the feeding of additives in the form of a plurality of strands to a cutter for making thermoplastic shaped articles.